The present invention generally relates to the controlled flow of ions through character forming means to form solid character electrostatic charge patterns on a dielectric surface. Specifically the invention relates to a non-impact electrostatic printer particularly useful for high-speed production of printed output in a data processing environment.
Increasing amounts of statistical and informational data have become instantly available through the rapid technological advancement and widespread application of data processing systems. This data has been found most useful when organized into printed forms. These forms or reports are appropriately distributed within businesses and organizations to serve as records, reference materials, and decision-making aids. Prior art printing devices utilized in preparing these reports primarily employ electro-mechanically actuated printing means such as solenoid actuated print hammers used in conjunction with constantly rotating print wheels and belts. These prior art devices accomplish printing by impacting the print hammers against a moving web and ink ribbon placed in juxtaposition to the character dies. The inertial characteristic and other mechanical properties of the prior art devices have limited their print rate to levels far below the rate at which contemporary computer systems can provide data. The electro-mechanical mechanisms employed in these prior art printers are often subject to failure and breakdown and result in the loss of the use of the printer to the computer system.
To overcome the deficiencies in speed and reliablility of impact printers, non-impact printing devices have been developed which employ electrostatic printing expedients. These prior art devices primarily employ one of two techniques for forming solid character charge images on a dielectric surface. The first technique comprises a movable dielectric web positioned between a plurality of solid character-shaped electrodes and a backing electrode. U.S. Pat. No. 3,234,904 issued to Wagner discloses an electrostatic printer of this type whereby selective pulsing of the solid character electrodes in coordination with a pulsed backing electrode deposits solid character charge images on a dielectric web. These charge images are subsequently toned and fused to accomplish permanent printing.
A second technique utilized in non-impact printing employs an endless stencil with character-shaped apertures formed thereon. The stencil is interposed between a plurality of discharge electrodes and a dielectric web. U.S. Pat. No. 3,314,360 issued to Forster discloses such an apparatus whereby clouds of ions selectively emitted from the discharge electrodes are shaped into solid character charge patterns by passing through the stencil. The charge patterns are induced to impinge on the dielectric web by attraction to an electrically modulated backing electrode which is positioned behind the web. Again, the charge images are toned and fused to accomplish permanent printing.
The prior art non-impact solid character electrostatic printers obviate many of the speed and reliability problems attendant with the electro-mechanical impact printers. However, there exist disadvantages inherent in these prior art non-impact solid character printers. One disadvantage of the solid character electrode devices is that the ionization breakdown voltage of the discharge electrodes is critically dependent upon the magnitude of the air-gap separation between the discharge electrodes and the backing electrode. A slight variation in electrode separation will result in a significant deterioration of the print quality. Variations could commonly arise from lack of uniformity in web thickness or improper printer adjustments.
Another problem common to prior art devices is a limitation on the printer design configurations because of the requirement that a backing electrode be positioned closely behind the print receiving web to attract the charge character patterns onto the web. Also, prior art devices which employ a backing electrode require complex circuitry to actuate the discharge electrodes and the backing electrode in proper synchronization. Such circuitry decreases the reliability of the prior art devices and significantly increases the cost of production and maintenance. Attempts to provide straight pulsed discharge electrode devices have proved inefficient because of the high voltage pulses required to form charge images on the image receiving surface.